Start her off with the basics- stringing necklaces is a great place to start! I know our Michael's offers some classes, so you could look into that for her. Watch for Michael's coupons to get supplies a little cheaper. Michael's also has bracelet/necklace kits with colored glass beads and seed beads- you would need to buy beading wire, crimp beads and some clasps to go with it, but she can make bracelets and necklaces with them, and it would be a good starting point to see if she likes it or not. A beading board might also be a good idea- it lets you lay out the beads in the pattern you want before you start stringing. I think Michael's has some at a reasonable price, if you use a coupon. As for sterling silver, use coupons to get some smaller pieces from michael's, like clasps and earring hooks. If you use a weekly 40% off coupon and buy one thing a week, you can get some pretty good prices on it.
Fire Mountain Gems is a great place for supplies at great prices, but can be overwhelming the first time you order from them, and it can be tempting to buy a lot from them because of the great prices (I have beads and stones from them that I've had for over a year that are still waiting to be used

). I would start her off with a beading class or a small kit from a local craft store, and then if she seems to really like it, order her some things from Fire Mountain. If you really want to get her swarovski crystal, though, Fire Mountain is the place to order them from- they have a great selection, and the best prices I've found so far.
When I was 12, I wanted to get into jewelry making because my grandmother and great-grandmother made jewelry, and begged for a kit from BJ's that I saw for Christmas. My mom thought it was just a passing thing, but got it for me anyway- and I'm so glad that she did. Here I am, 8 years later, and I make and sell jewelry at craft fairs, and am hoping to go to jewelry design school in the next few years, after I get my PR degree. I know that isn't the case with every little hobby someone picks up, but because of it, I feel like its worth it to let people experiment with different hobbies.
If you have any specific questions about supplies or anything, feel free to PM me and I'll do my best to answer them!